1. Field of the Invention
The present invention relates to a device for measuring distances between an optical element of large chromatic aberration and an object, white light emitted by a light source being imaged on the object by the optical element, said object reflecting a fraction of the incident light which is imaged on a spectral-dispersive apparatus.
2. Background Art
A normal glass lens may be used to focus white light, emerging from an aperture (see, e.g., FIG. 1). A rough look at the screen shows an image of the aperture, if the screen is located at the correct distance d from the lens. If the experiment is repeated with filters of different colors positioned between the lamp and the aperture, one finds that different distances d have to be chosen to get sharp images of the aperture for the different colors. This is well known in optics ("chromatic aberration").
White light is a mixture of light of different colors. If the experiment shown in FIG. 1 is repeated more carefully with white light, we will find the sharp image of the blue component at the distance d(blue) and the sharp image of the red component of the white light at a distance d(red) (see, FIG. 2). The colors red and blue have been chosen as examples, because they form the long and the short wavelength limit of the visible spectrum of light. The other colors out of the white light, which are not shown in FIG. 2, have their focus somewhere between the red one and the blue one. Actually, at any position between A and B of FIG. 2, there is one color, which has its sharp image exactly there.
A screen, positioned at B, will show a sharp image of the red component. This means, that all the red light is concentrated in a small circular area (see, FIG. 3). All the other colors are distributed over a much larger area. This means that a spectral analysis of the central spot's illumination will show that it is red because the illumination density (amount of light per unit area) is highest for the red light and lowest for the blue light.
At a different position of the screen, another color will dominate the center. Thus, the color of the central spot is correlated to the spacial position of the screen and an analysis of this color will give the position of the screen (i.e., relative to the lens and light source). The "screen" can be almost any object that scatters or reflects light. Thus, the analysis of the central color becomes a measurement of the distance between the lens and arbitrary objects.
All the effects described above are difficult to observe with normal lenses, because normally lens makers try to make these effects as small as possible. On the other hand, there are optical components, which show these effects strongly. One way to enhance these effects is the choice of special glasses for the lens. Another way is to use a "zone plate" instead of a lens. A zone plate is a diffraction grid with especially spaced circular grooves. The refracting power is proportional to the wavelength of the incident light. Thus, for example, the focal length of a f=10 cm zone plates differs by some centimeters for red and blue light.
In the U.S. Pat. No. 4,585,349 it has been proposed to optically measure the distance of a point of the reflecting surface that is located on the optical axis of the measuring device. This measurement is based on focusing of polychromatic light, each focal distance being characteristic of the respective wavelength. By trapping the light reflected by that part of the surface to be measured which is located on the optical axis, an intensity maximum of the wavelength is measured which is characteristic of the distance of this part of the reflecting surface. To perform this measurement, the reflected light is transmitted to a diffraction grating which scatters it according to its various wavelengths, and a series of optical measuring cells are used to determine the wavelength of the highest light intensity received. The focal distance of this wavelength corresponds to the distance between the holographic lens for focusing the polychromatic light and the reflecting surface. This solution permits measurement of the distance of a point, but not imaging of the surface profile, which substantially limits its area of application.
In addition, a conventional optical depth measurement method is known which is based on the effect of the parallax or of triangulation, according to which a light bundle is projected on an object. A detector observes this point or this illuminated line at different angles. The effect of the parallax permits calculation of the distance of the surface or of its profile.
The drawback of this method is to be seen in the fact that either the angle of light or the angle of observation or both must be oblique, which limits the depth of the holes or grooves that can be observed.
The basic ideas of chromatic sensors have been described above, but this still leaves unsolved the problem of analyzing the color of the central spot. One possibility is to use the lens (or zone plate) to monitor the central spot. This can be done by inserting a semitransparent mirror between the aperture and the lens (see, FIG. 4). One half of the light, emerging from the aperture, traverses the mirror and reaches the screen (the object). The light, scattered by, or reflected from the object, is then collected by the lens (or zone plate) and directed back towards the mirror. One half of the returning light passes the mirror towards the aperture and is lost. The other half is reflected downwards onto the spectrum analyzer.
There is an aperture in front of the spectrum analyzer which allows only the light of the central spot to pass (naturally, if all the light which is reflected by the object were summed and analyzed, the spectrum would be that of the white light emitted by the lamp, but the present invention definitely does not perform in this way--rather, by allowing only the light from the central spot to enter the analyzer, the wavelength-dependant distance measuring capabilities discussed above with reference to FIGS. 1-4 are preserved). For color analysis, nearly every optical spectrum analyzer is suitable.